As described in U.S. Pat. No. 4,500,523 and elsewhere, the myometrium contracts spontaneously throughout gestation. This activity has been confirmed experimentally in several species, including humans, by measuring increases in intrauterine pressure (IUP) (also sometimes referred to as intra-amniotic pressure (IAP) or changes in myometrial electromyographic activity (EMG). Continuous IAP and EMG recordings in chronically catheterized sheep show the presence throughout most of gestation of "contractures", EMG events of long duration (5-7 min.) that produce a small increase in IAP. The pattern of duration and frequency of activity events changes as labor approaches; Nathanielsz et al., Am. J. Obstet. Gynecol., 138:653 (1980); Krishnamurti et al., J. Reprod. Fert., 64:59 (1982).
It is further known through the study of electromyographic activity (EMG) in pregnant rhesus monkeys that at least two types myometrial activity occur during pregnancy. One of these, sometimes named Type I and associated with labor and delivery, is characterized by frequent bursts (e.g. 10 to 45 per hour) of electrical activity that last from 0.5 to 1.2 minutes. The other type of EMG activity observed, sometimes named Type II, and associated with spontaneous myometrial contractions during the normal development non-labor and delivery stages of pregnancy, consists of fragmented series of discharges that last 2 to 15 minutes and occur up to six times an hour. The frequency of Type I EMG events when present generally show a 24 hour pattern of low amplitude with a maximum at night. This daily pattern is amplified during the 8-10 days preceding delivery.
The pregnant rhesus monkey has been extensively investigated to study changes in myometrial activity and responses to various physiological and pharmacological agents. In this species myometrial electromyogram (EMG) electrodes and sensors for measuring intrauterine pressure (IUP) can be placed at the beginning of the last third of gestation or even earlier. Several groups of investigators have shown varying degrees of activity at different times of the 24 hour day during various stages of gestation [Harbert, Am. J. Obstet. Gynecol., 129:401-408 (1977); Harbert et al, Am. J. Obstet. Gynecol., 138:686-696 (1980); Taylor et al, Am. J. Obstet. Gynecol., 146:557-567 (1983); Ducsay et al, Am. J. Obstet. Gynecol., 145:389- (1983); Nathanielsz et al, In "Animal Models in Fetal Medicine" Vol. III Ed. Nathanielsz, Perinatology Press, Ithaca, New York pp 110-160 (1984)]. These cycles of myometrial activity have a clear temporal relationship to the circadian hormone changes in maternal and fetal plasma.
In pregnant sheep and monkeys, epochs of myometrial activity differing in duration and intensity have been described; Nathanielsz et al, In: "The Fetus and Birth" Ciba Foundation Symposium 47 Amsterdam, Elsevier pp 73-91 (1977); Nathanielsz et al, Am. J. Obstet. Gynecol., 138:653-659 (1980). One method of assessing the nature of activity at any moment is to characterize duration and amplitude of each individual event of myometrial EMG activity or IUP change either together or separately. In the sheep we have described computer based methods for the determination of duration of individual epochs of activity; Figueroa et al, Am. J. Obstet. Gynecol., 151:524-531 (1985). A preliminary description of similar event or epoch counting methods for use in the pregnant rhesus monkey has been presented to the Physiological Society, Cambridge, England, July 1984. In sheep, throughout the majority of gestation the most common epochs of myometrial activity are contractures, i.e. Type II activity, which we have defined as long duration epochs of increased IUP or EMG activity lasting more than three minutes [Jansen et al, Am. J. Obstet. Gynecol., 134:766-783 (1979); Nathanielsz et al, 1980, supra; Figueroa et al, 1985, supra; Harding et al, Am. J. Obstet. Gynecol., 142:448-457 (1982)]. At labor and delivery, myometrial activity shows a contraction type pattern during which the epochs of activity are shorter than two minutes, i.e. Type I activity. To obtain a clear impression of the type of myometrial activity under different physiological conditions, contractures need to be distinguished from labor and delivery contractions. Assessment of myometrial activity as Montevideo Units or area of activity under a pressure curve does not permit such a distinction since by using these methods one loses important information on duration and frequency of individual epochs of activity.